The word 'volcano' comes from 'Vulcan', the name for the Roman god of the layers of stone which make up the Earth's crust are not all of the same thickness. Under the layers the temperature is so high that the rock can melt and become molten rock: magma.

The magma lies in reservoirs which move with the land's movement. As they move, some gas escapes and puts pressure on the inside of the Earth's crust. If this immense pressure is on a piece of crust which is thin, or is moved by an earth tremor, the gas can escape through a hole. There is an explosion. The magma moves into the breach towards the Earth's crust, cools and solidifies. When it solidifies it is called lava. Progressively a cone forms around the hole, making a volcano. Sometimes a volcano continues to erupt for years. Large clouds of smoke, ash, rock and magma escape. If there isn't much gas, the subterranean pressure decreases, and the eruption dies out. In the cone the magma cools and the opening is resealed. But sometimes, years later, the gas pressure builds up to such a pitch that a new eruption is inevitable, and the process starts again.